Validate configured groupings
Validate configured groupings

New grouping criteria must be configured in [groups.yaml](https://github.com/alphagov/finder-frontend/blob/736bb24164cd76f00710d7c0ea622948c9711a44/app/lib/brexit_checker/groups.yaml)
and [group_validator](https://github.com/alphagov/finder-frontend/blob/736bb24164cd76f00710d7c0ea622948c9711a44/app/lib/brexit_checker/validators/group_validator.rb)
before being shipped, otherwise things start to error.

This adds a test that validates the live action.yaml file to ensure that
the groupings specified in there can actually be honoured by the app.

alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend,alphagov/finder-frontend